September 4, 2007

Professor Researches Human Genome

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According to a recent Cornell study, there may be something other than time that separates us from our deep ancestral backgrounds. In June of this year, several Cornell professors published a report in the Public Library of Science indicating that as much as 10 percent of the human genome has been subject to adaptations at the molecular level. They assert that the genome, the genetic context of an organism, has been altered by natural selection.
Darwinian principles are far from outdated. In fact, according to senior author of the paper and Prof. Rasmus Nielson, biological statistics and computational biology, much of the natural selection occurring happened after the major human lineages split up — within the last 100,000 years, which is considered recent in the context of world history.
“The research is an exploration into the hidden secrets of the human genome,” Nielsen said.
The researchers specifically set out to study the type of selection that occurs in response to changing environments, as well as the selection that might occur due to rising pathogenic threats. They felt gaining this type of information would better allow them and the rest of humankind to understand more about evolution.
“In general, there exist two opposing forces,” said Prof. Carlos Bustamante, biological statistics and computational biology.
“As a population gets bigger, selection gets stronger,” he said.
With this basic principle in mind, the researchers discovered 101 regions of the genome where natural selection might have played a hand. According to Nielsen, the researchers used data that compared more than a million variable positions of the human genome, and looked for differences among those of varying ethnic origin. Researching this way allowed them to see how different occurrences of selection affected different populations, including those of African-American, European and Chinese descent.
“When adaptation happens in the genome, it leaves a characteristic pattern behind, which gets referred to a lot as the signature of selection. It is only good so long as it is unique,” said Prof. Scott Williamson, biological statistics and computational biology.
According to the published paper, the researchers supported the hypothesis that as the ancestral humans left sub-Saharan Africa, the selective pressures of their new environments forced a heightened rate of population-specific selective adaptations. This precisely justifies the trend explained by Williamson that the Chinese and European-American populations seemed to experience a lot more adaptation than those of African-American descent. It was the former two populations that left the African area, thus they were the ones that had to adapt to new atmospheres.
“Much of the selection might be due to immune and defense related functions, but we also found evidence for selection in a number of other systems — for example, in genes related to skin pigmentation,” said Nielsen.
In their analysis of skin pigmentation, an obvious distinction among these ethnic groups, the researchers were able to locate several candidate genes responsible for this difference in color.
They further found that different ethnic populations have different genes taken to be ‘selected.’
Summating the results construed from various candidate genes, the faculty members were able to suggest that adaptation to local environments drives the evolution of human skin pigmentation.
Other than skin coloring, the researchers noted in the paper that they also found a recent adaptation in a cluster of olfactory genes, which relate to the sense of smell, especially in the African-American population. Among the European-Americans, the group found that hair morphology candidate genes had consistent signals of recent adaptation.
The allele that allows for adults to metabolize lactose was also a lot more common in the European population.
“[The lactose trend] implies that people who could metabolize lactose were better able to take advantage of their resource,” Williamson said.
Williamson further explained that the trend was more common in areas that have a history in dairy farms, such as those in which the European population resided.
The research allowed for a better understanding of the human genome and of human evolution. The researchers feel that the study makes the functional differences between and among the ethnic populations noted above much easier to grasp.
“We cannot predict future evolution,” Nielsen said. “But it is likely that the way selection is acting has changed drastically since the invention of modern health care, modern hygiene, etc.,” he said.
Several other Cornell faculty members that collaborated on the research include Prof. Melissa J. Hubisz, biological statistics and computational biology, Prof. Andrew G. Clark, molecular biology and genetics and Prof. Bret A. Peyseur, molecular biology and genetics.